1. Field of the Invention
This invention relates to sound detection systems.
More particularly, the invention relates to a system for detecting the siren of an approaching emergency vehicle and for controlling the traffic lights at an intersection to facilitate passage of the vehicle through the intersection in a desired direction.
In a further respect, the invention relates to a siren detection system which accurately determines the frequency of sound from a siren by counting pulses that indicate the frequency of the sound, by determining the elapsed time necessary to count a selected number of pulses, and by utilizing the elapsed time and number of pulses counted to determine the frequency of sound emitted by the siren.
In another respect, the invention relates to a siren detection system which identifies the presence of a siren by taking a sequential series of samples of sound frequencies being emitted by the siren and by determining whether the series of samples includes a selected number of sequential frequency samples having rising values and includes a selected number of sequential frequency samples having falling values, the specific value of each frequency sample being irrelevant as long as each value falls within a broad frequency band occupied by sound frequencies produced by conventional sirens.
In still a further respect, the invention relates to a siren detection system which can be adjusted to be activated when an approaching siren is at one of a selected number of different distances from the detection system.
In yet another respect, the invention relates to a siren detection system in which the volume of sound from an approaching siren is determined by averaging the amplitude of each of a selected sequential series measured frequency values which represent the sound produced by the siren.
2. Description of the Related Art
Systems for detecting the siren of an approaching emergency vehicle are well known in the art. See for example, U. S. Patent Nos. 4,759,069 to Bernstein et al., 4,625,206 to Jensen, and 4,806,931 to Nelson. The Bernstein et al. patent provides a useful summary of prior art systems. The system described in the Jensen patent, require the sequential or cascading detection of specific signal frequencies. The system in Bernstein et al., identifies a siren by using a bank of stepped, narrow band frequency filters to detect a series of frequencies each in a specific range. This predisposition toward premising the detection of a siren on the identification of a series of specific frequencies appears central to the prior art. In the Nelson patent, a series of frequency samples is fit to a specific algorithm which defines the sound pattern of the siren detected by the system.
In practice, attempting to identify and correspond a sequential series of sound frequency sample values to a specific selected series of reference frequency values appears difficult to do, probably because of the presence of buildings and of other surfaces which reflect sound, of systems to accurately extraneous noise, and of other unknown factors which affect the ability of prior art systems to accurately detect specific sound frequencies. In particular, attempting to make the system described in U. S. Pat. No. 4,806,931 to Nelson function is not readily accomplished. Obtaining a sequential series of sound frequency sample values and fitting the values to a reference algorithm or curve in the manner illustrated in FIGS. 3A to 3C of the Nelson patent was found difficult.
Also central to the prior art is the determination of the frequency of sound from a siren by counting direct current pulses derived from sound received from the siren. The number of pulses counted during a set sampling period of time is, as is well known in the art, used to determine the frequency of the sound. The drawback to such counting techniques is that only "complete" pulses are counted during the set sampling period of time. Once the sampling period of time expires, counting stops. When a relatively small number of pulses is detected and counted during a sampling period, a large error in counting can occur. If, for example, three pulses are counted during a sampling period, it is possible that a fourth pulse was about to be counted at the instant the sampling period terminated. The failure to count this fourth pulse would introduce an error of about 25% into the count samples
Accordingly, it would be highly desirable to provide an improved siren detection system which would count with improved accuracy the number of pulses derived from sound emitted by a siren and which would, without requiring the fitting of a group of sequential frequency sound sample values to a specific sequential series of reference values, permit the presence of a siren to be determined from the group of sample values.
Therefore, it is a principal object of the invention to provide an improved sound detection system.
A further object of the invention is to provide an improved system to detect an approaching emergency vehicle siren, to identify the direction of approach of the emergency vehicle, and to control the traffic lights at an intersection to facilitate passage of the emergency vehicle through the intersection.
Another object of the instant invention is to provide an improved siren detection system which can identify the presence of a siren from a sequential series of sound frequency sample values without requiring that the sound frequency sample values fit a selected series of specific reference values.
Still a further object of the invention is to provide an improved siren detection system which more accurately counts the number of direct current pulses derived from siren sound received by the detection system.